We report on a locally recurrent vulvar tumor in an 80-year-old woman that we believe represents the first example of malignant transformation of an angiomyofibroblastoma. The tumor was predominantly a typical angiomyofibroblastoma, composed of epithelioid or oval cells with eosinophilic cytoplasm that tended to cluster in small groups and around blood vessels. These areas merged imperceptibly with a high-grade sarcoma that resembled a myxoid malignant fibrous histiocytoma. The tumor cells in the benign areas were diffusely immunoreactive for vimentin; many cells were positive for smooth muscle actin, and focal positivity for muscle actin and desmin was observed. The tumor cells in the sarcomatous areas were diffusely positive for vimentin, but negative for smooth muscle actin, muscle actin, and desmin. No staining for keratin, S-100 protein, or CD34 was noted. Ultrastructural examination of the sarcomatous area showed that the cells had the features of fibroblasts. All previously reported cases of angiomyofibroblastoma have exhibited banal histologic features and have behaved in a benign fashion. This case shows that these tumors may rarely be associated with a malignant component, and the designation "angiomyofibrosarcoma" may be appropriate in such cases.